Market adoption of wireless LAN (WLAN) technology has exploded, as users from a wide range of backgrounds and vertical industries have brought this technology into their homes, offices, and increasingly into the public air space. This inflection point has highlighted not only the limitations of earlier-generation systems, but the changing role WLAN technology now plays in people's work and lifestyles, across the globe. Indeed, WLANs are rapidly changing from convenience networks to business-critical networks. Increasingly users are depending on WLANs to improve the timeliness and productivity of their communications and applications, and in doing so, require greater visibility, security, management, and performance from their network.
As enterprises and other entities increasingly rely on wireless networks, monitoring and management of the components implementing the wireless network environments become critical to performance and security. Heretofore, it has not been recognized how important visibility into all layers of the network protocol is to optimization of network manageability and user performance in wireless LANs (WLANs). Unlike centrally-managed cellular wireless systems, known WLAN solutions use distributed access points to act as bridges between the wired infrastructure and the wireless clients, removing all physical and wireless media access protocol information from the protocol frames that are passed onto the infrastructure network. This results in uncoordinated handoffs of wireless clients moving between access points. An uncoordinated system of access points makes it difficult to manage a large number of access points, because there is no point of coordination. For example, known prior art wireless network systems such as conventional 802.11 systems provide the initial handshaking, access authentication and access association at a remote node without attention to overall network loading and signal quality.
This type of distributed architecture creates many problems affecting network management, mobility, and performance. Since each wireless LAN access point is a separate managed device, a distributed architecture in general introduces many new managed elements in the network without sufficient attention to their global effects. Since the access points act in their own self-interest and are not aware of the actions taken by surrounding access points, they handle mobility (e.g., handoff actions) as a local event, which significantly increases latency.
U.S. application Ser. Nos. 10/155,938 and 10/407,357, identified above, disclose a hierarchical wireless network architecture that optimizes network management and performance of a relatively autonomously-managed WLAN. According to the system architecture, a central control element (wireless switch) manages and controls one more access elements. These light-weight access elements perform real-time communication functions, such as transmission of beacon packets, data transfer and acknowledgements, while the central control element manages the connection between the access element and one or more wireless client devices.
A vital function to the operation of WLANs is roaming—i.e., the handoff of a wireless client from one access point to another as the client roams about the coverage area provided by the wireless network infrastructure. During a handoff event, a wireless client essentially abandons its connection with one access point and establishes a new connection with another, resulting in a small period without connectivity and therefore possible packet loss. A handoff event can be divided into three phases: 1) scanning/probing, 2) re-association, and 3) state information transfer. When the signal quality between an access point and a client degrades, the client sensing that connectivity has been lost initiates a handoff by scanning available RF channels for access points with which to associate. After selecting an access point identified during the scan, the client attempts to associate with the selected access point.
In 802.11 WLANs, a typical handoff may involve an exchange of messages between the prior and new access point to accomplish a transfer of physical/link layer connectivity between one access point and another access point. The message exchanges may include the transfer of connection state information between the prior access point and the new access point as well. The connection state information may involve the exchange of credentials and other state information between the access points. For example, the IEEE 802.11F specification provides a recommended general framework for the exchange of connection state information between access points during a client handoff. For example, when a wireless client discovers a new access point, it transmits a re-association message, including the Basic Service Set Identifier (BSSID) of the old access point, to the new access point. The new access point uses some mechanism (not specified in the 802.11F specification) to resolve the address of the old access point and transmits a request for the connection state information of the client. In hierarchical wireless networks, the handoff can occur between two access points managed by the same wireless switch, or between access points managed by different wireless switches. In the former case, the transfer of connection state information between access points is either not required or greatly simplified, as this connection state information is maintained by a common wireless switch. Client handoffs that implicate two wireless switches, however, may require the exchange of wireless connection state information between the wireless switches. While it is possible to use the roaming protocol set forth in the 802.11F specification to transfer state information between the switches, the mapping of BSSIDs to wireless switch addresses presents configuration overhead, especially for large scale deployments. For example, the draft 802.11F specification discloses that Remote Authentication Dial In User Service (RADIUS) servers may be used to provide the mapping between the BSSID and the wireless switch. Furthermore, the 802.11F specification does not address the situation where a client attempts to re-associate with two switches in alternating succession, or with multiple switches, as it roams about the wireless network environment and the subsequent issues that arise to determine which reassociation is to be acted upon. Furthermore, the process flow set forth in the draft 802.11F specification introduces latency by requiring the access points to exchange connection state information before allowing the client to gain network access.
While the foregoing systems work for their intended objectives, the latency associated with exchanging state information, such as authentication credentials, between access points may be too large where desired QoS levels are high. Accordingly, a need in the art exists for methods, apparatuses and systems that reduce the latency associated with roaming in wireless networks.